Research to evaluate conditions for the safe and effective use of current pesticide chemicals and to develop better pesticides with improved selectivity continues to focus on their metabolism, photodecomposition and mode of action. The major emphasis is on synthetic pyrethroid insecticides, which have recently entered major use in agricultural pest control. Chemical studies involve photodecomposition, synthesis of metabolites and photoproducts, and spectroscopy and analysis of the pyrethroids and their degradation products. Toxicological investigations on pyrethroids concern their metabolism in various organisms and enzyme systems, their synergistic interactions with other pesticides, and their neurophysiological effects. Studies on phosphorylating and carbamoylating agents consider those actions of organophosphorus and methylcarbamate pesticides and related compounds which are not attributable to acetylcholinesterase inhibition, e.g., the biochemical lesions leading to teratogenic and/or delayed neurotoxic effects in vertebrates. Mode of action studies are in progress on S-alkyl phosphorothiolate toxicants. Studies on novel or poorly understood target sites examine the mode of action of antidotes for herbicide injury to crops, herbicides and their metabolites that inhibit photosynthesis, and inhibitors of insect chitin synthesis and chitin synthetase. Structure-activity considerations are important in these investigations. Studies on reactions of mutagenic and possibly carcinogenic pesticides and their degradation products attempt to identify the actual mutagenic components of pesticides and their metabolites and photoproducts. S-Chloroallyl thiocarbamate herbicides and pyrethroids are emphasized in these studies. It may be possible to avoid the serious implications of mutagenic and carcinogenic lesions by changes in manufacturing methods and purification procedures or by slight structural modifications on the pesticides.